This must be the definitive list of ideas on how to use Twitter in Education. In fact, it is a very good overview of Twitter in general with some very specific ideas for devolping the concepts behind the twitter community into something useful for students, teachers and their relationships.
Have a look: http://tinyurl.com/cnq89x8
If you have experience using Twitter with students would love to hear from you. It is not something I have really tried yet. I have to say I am a little behind and not completely convinced on using social networks with my groups as they seem to have so little time outside of class to get involved. Maybe that is the problem, I just need to make the experience useful and attractive enough.